Automotive vehicles having independent suspensions are generally equipped with stabilizer bars to reduced inclination or roll of the vehicle bodies during vehicle maneuvers. The stabilizer bar is usually connected between the suspension arms of the vehicle wheels. When the left and right wheels are in similar positions with respect to the suspensions, the stabilizer bar does not twist so that the suspensions are mutually independent. When one of the left wheel and the right wheel passes over a bump on the road surface, or when the vehicle turns and, thus the left wheel and the right wheel assume considerably different positions with respect to the suspensions, the stabilizer bar is twisted. This twisting motion induces a torsional resilient force for affecting the handling and ride performance characteristics of the vehicle wheels.
It is desirable that the torsional resilient force of the stabilizer bars can be adjusted in accordance with ride and handling conditions of the vehicle. Specifically, it is desirable to reduce the torsional rigidity during straight travel of the vehicle and to increase the torsional rigidity during turning of the vehicle. The reduced torsional rigidity enhances the ride and handling characteristics of the vehicle wheels while the increased torsional rigidity enhances the handling and ride characteristics of the vehicle.
Certain vehicle active tilt control systems include front and rear stabilizer bars which are adjustable by front and rear hydraulic actuators placed in lieu of the stabilizer bar linkages. The actuators are movable in first and second opposing directions for adjusting vehicle body active roll movement to compensate for vehicle roll.
The smoothness of the actuator motion is very important for a comfortable ride. Smooth motion is a motion without discontinuities in actuator projectories. "Monotonic" motion is defined as a change in the velocity vector angle in only a single direction. In other words, as an actuator is moving in a certain direction to torque the stabilizer bar, the velocity gradient of the actuator should not change directions.
When the velocity vector angle only decreases during the actuator motion, the motion is considered high quality motion. On the other hand, motion in which the velocity vector angle is both decreasing and increasing during a directional movement, the motion is considered low quality. These two situations are depicted in FIGS. 1 and 2. In FIG. 1, the velocity vector angle .theta. is always positive, which is indicative of a high quality motion. On the other hand, in FIG. 2, the velocity vector angle .theta. sometimes reverses in a negative direction, which results in unacceptable motion of quality.
In many motion control systems where linear or rotary actuators are implemented, it is also desirable to have decreasing velocity toward the end stop of the actuators. Velocity near zero at the end stop would be the ideal case.
The active tilt control system is not exempt from these rules. If the actuator velocities are not decreasing towards the end stop, it is likely that the transient roll angle will overshoot. Even if the overshoot is not present, roll motion may be uncomfortable for passengers. Another result of high speed of the actuator near the end stop may be caused by violent acceleration changes when the actuator slams against the end stop. The acceleration in this case is a scaled version of the supply pressure. If the pressure transient within the actuator chambers is not smooth and has break points or discontinuities or high frequency oscillations, the actuator's elements will suffer damage much earlier in life, which reduces component reliability.
Discontinuities in actuator motion are considered break points that represent sudden change in the actuator velocity angle. This may result in the actuators speeding up at the end stop, as shown in FIGS. 3a and 3b. A comparison of the front and rear actuator motion illustrated in FIG. 3A and 3B will show that the front and rear actuators are not synchronized. Accordingly, the front actuator bottoms out first, which sends pressurized fluid into the rear actuator, thus, accelerating the rear actuator against its end stop. This is a highly undesirable situation.
Various automotive suppliers have suggested different proposals for smoothing motion of the actuator near the end stops and also reducing velocity near the end stops. Such proposals, typically, comprise numerous hydraulic accumulators or special spool valves for distributing flow between the front and rear actuators. However, these proposals result in expensive assemblies.
Accordingly, it is desirable to provide a vehicle active tilt control system in which actuator motion near the end stops is smooth, and velocity is near zero near the end stops. It is further desirable that such a system be inexpensive.